Horseshoe crabs breathe through their gills like fish do yet their gills are on the underside of their bodies. One can only see them when you pick one up and turn it over. They looked like layered fans. Their gills allow them to breathe, unlike fish, both in water and on land.

And yes, they have a cardiac system to move that blue blood around. (See earlier post). One of the main criticisms of their bleeding by pharmaceutical companies is that they insert the needles near the hinge on the horseshoe crab which is also right where their main cardiac is located. This would be like having a large needle jammed in your neck or chest and your blood sucked out. We don’t know for sure if they feel pain but they must if they have a brain and neurological system.